This disclosure is directed to a method and apparatus for servicing a wellhead assembly. A wellhead assembly is sometimes known as a Christmas tree and is the equipment which is attached at the top of a well, normally standing above the ground and looks like a Christmas tree. The Christmas tree construction typically involves an upstanding set of control valves, flanges and other couplings which enable connection of the well to any number of flow lines. For instance, the equipment typically joins to a gathering line. There is also a connection to the casing so that the Christmas tree is raised.
A well is first drilled with no assurance that the well will be a producing well. Typically, the well is drilled to a specified depth, and perforations are placed into the side of the partly completed well into formations to obtain production. If the production is adequate, the well is cased and cemented. At that stage of proceedings, if the production flow rate is adequate, then the well will be completed. A completed well normally includes a production tubing string which extends from the producing formation up to the surface. The production tubing string is located on the interior of the casing. The casing extends to the surface around the production tubing string. The combination of the casing and the external cement layer prevents leakage of artesian water along the well borehole on the exterior. If the casing were not cemented in place, it might readily permit salt water to migrate upward or downwardly into formations where salt water is not desirable. The casing is longer than the tubing string because the tubing string may not extend to the bottom or total depth of the well that is actually cased. The production tubing string is located on the interior of a production casing string. Both are several thousand feet in length. In addition, there typically is a surface casing which provides some isolation at the immediate area of the wellhead so that an annular space extends downwardly on the inside of the surface casing perhaps only 200 feet to as much as 2,000 feet in depth.
The several pipes defined above are the production tubing, the producing casing, and the surface casing all support the Christmas tree or wellhead assembly. The Christmas tree is affixed at the exposed upper end of the concentrically arranged pipes just mentioned. The Christmas tree normally includes a wellhead assembly which supports multiple laterally extending valves which are installed with suitable mounting flanges. The Christmas tree can have a variety of valves and fittings attached to it depending on the particular requirements. In the present disclosure, a relatively simple and common producing well is described i.e. a well having one production tubing concentric within the casing. The wellhead attached to the casing extends typically between 5 and 15 feet in height. In some instances, it can even be taller depending on the requirements on the particular wellhead assembly.
There are any number of valves including production casing valves and production tubing valves attached to the wellhead assembly. These valves are periodically opened and closed. Over time, wear in the use of these valves accumulates, and leakage will occur. This requires service including the occasion of removing one or more of the valves, changing out the valve seats, replacing the valve seals, dressing the valve elements and other of steps. All of these steps require that the well be interrupted so that flow is no longer permitted. After a well has been in service for a number of years, the formation drive pressure maybe reduced. If the well is shut-in by the wrong technique, it can kill the well temporarily, perhaps damage the formation, and thereby prevent production flow from the well when the wellhead has been serviced. It is highly undesirable to stop the flow using certain approaches. One technique which is highly detrimental is filling the production tubing string full of a weighted fluid such as drilling mud. If the pressure balance becomes reversed, the production fluid can be forced into the producing formation, thereby filling the formation in an annular region near the well, plugging the perforations and preventing subsequent flow.
There are other techniques for killing a well which are less detrimental. The present disclosure sets forth one such approach. Competing approaches are set forth in the referenced patents which describe the status of the prior art.
U.S. Pat. No. 2,552,901 of Miller sets forth a refrigeration system utilizing the container 19 filled with ice water or dry ice at 18. The freezing procedure extends to the interior so that water will form a static ice plug on the interior. This freezes on the interior of the casing 7. It is located below the Christmas tree.
U.S. Pat. No. 3,738,424 describes a production procedure, especially for a producing well, to get control during a blow out, and features the long central tubing string shown in the drawings which is supported with a number of radial disk 30 attached to the tubing 31. In FIG. 3, a flow path is shown from the bottom valve, into the well, through the disk, and back out through the outlet to the duplicate upper valve. At column 4, it describes the flowing liquid nitrogen introduced through the tubing 60. That provides the freezing. Freezing is imparted to water flowing the tubing string T.
U.S. Pat. No. 4,203,472 of Delaney shows a jet mechanism in the nozzles 28 shown in FIG. 2 of the drawings which are directed radially inwardly to form a frozen plug. Freezing occurs on the exterior of the pipe 26.
U.S. Pat. No. 4,372,378 of Powers sets forth a special joint to the installed during drilling to prevent a blow out. According to this disclosure, the special joint or session is installed so that nothing is done during regular drilling. When there is a blow out, a refrigerated fluid is delivered to flow in corporation with a set of flow deflectors which become active during the blow out. This enables freezing during reverse flow typified by a blow out.
U.S. Pat. No. 4,396,031 of Peterson is a simple two pipe enabling water to be added. With subsequent cooling in the flow, freezing can be accomplished. Presumably, it is intended to be used to control a blow out, see column 1.
U.S. Pat. No. 5,125,427 of Cantu sets forth a specific additive. The organic material includes identified alcohols or paraffins. This is used as an injectant. The specific location of the injectant is not shown in the drawings (there are none) and it apparently involves use at any location.
The present disclosure sets forth both a method and apparatus for servicing a Christmas tree or wellhead assembly in a fashion which does not harm the well. More importantly, it is a procedure which can be carried out with great safety. When removing parts of the Christmas tree, there is the risk of escape to atmosphere of oil, gas, water or any mix thereof. The present disclosure plugs the well in a fashion which enables service personnel to carry out any necessary servicing step at the wellhead. Indeed, servicing can include removal of portions of the Christmas tree. This process particularly enables portions to be removed and replaced.
Briefly summarizing, the present disclosure sets out a method and apparatus for servicing. The servicing approach utilizes a tub or bucket which is assembled around the outermost surface casing and which is filled with a cold liquid inside of a liner seal. The liners serves as a seal in the bucket. Moreover, liquid is added which preferably cools to a very cold temperature, and pieces of dry ice are dropped into the liquid.
In addition to that, access is gained to the one or more annular spaces on the interior of the bucket. In that region, a small trickle or spray of water or a gel fluid subject to freezing is introduced. The water is frozen, thereby plugging the annular space with an ice plug. The ice plug is built from the exterior inwardly. The ice plug completely blocks fluid flow in the annular space. This is done for every annular space, finally positioning a surrounding ice plug about the production tubing string. At that point in time, the Christmas tree can be opened, disassembled, and servicing can then be carried out. Examples will be given.